Though exclusively acoustic methods for wind profiling and the like have a long history, Coulter & Kallistratova in their 1999 article “The Role Acoustic Sounding in a High-Technology Era” [Meteorol. Atmos. Phys. 71, 3-19] show that these methods have not lived up to their promise. This appears to have been largely due to an inability to achieve an adequate signal-to-noise ratio [s/n].
Sodars for atmospheric sounding have almost universally employ short (millisecond), single-tone high power pulses, multiple receivers and simple timing circuits to determine the sequence of echoes at different receivers needed to deduce the height of various discontinuities in the atmosphere. U.S. Pat. No. 2,507,121 to Sivian [1950] disclosed a method for measuring the height of atmospheric discontinuities that involved sending such a pulse vertically into the atmosphere and, after cessation of the transmitted pulse, detecting vertically returned echoes using two similar receivers located near the transmitter. In the embodiment of most interest, the first receiver is shielded against receiving echoes but the second is not and the two receivers are connected so that their outputs are opposed and the net signal can be displayed on an oscilloscope. In the event of a normally returned echo, a pip is displayed because only the second receiver detects a signal. However, in the event of local noise such as a gunshot both receivers detect the same signal and no pip is displayed.
U.S. Pat. No. 3,889,533 to Balser [1973] disclosed an ‘acoustic wind sensor’ in which an acoustic transmitter illuminates a cylindrical column of air by either CW (continuous wave) or pulsed signals and remote receivers are pointed at narrow or broad portions of two or more sides of the side column to detect acoustic energy scattered laterally therefrom. The Doppler components of this scattered energy are then extracted to determine wind velocity at the various heights. In order to observe a portion of the illuminated column, which is—say—1000 m from the ground, the receivers need to be spaced from the transmitter by a roughly similar distance. Also, significant spacing is needed to sufficiently attenuate the direct signal [sometimes called the ‘zero Doppler’ signal] from the transmitter. An application of this system to the detection of persistent vortices near runways was disclosed in U.S. Pat. No. 3,671,927 to Proudian and Balser.
U.S. Pat. No. 3,675,191 to McAllister [1972] disclosed the use of four adjacent arrays of acoustic transducers capable of being used as speakers and microphones, the arrays being aligned with the cardinal points of the compass and being shielded from one another, except at their upper faces. Short acoustic pulses were transmitted vertically upwards and the relative timing of the returned echoes at each of the four arrays gave the height and bearing of wind layers. [It might be noted that the physics of acoustic sounding was well documented in 1969 by McAllister and others in “Acoustic Sounding—A New Approach to the Study of Atmospheric Structure” in Proc. IEEE Vol. 57, 579-587.] U.S. Pat. No. 4,558,594 to Balser disclosed the use of an acoustic phased array capable of directing successive pulses in different directions, the echoes from one pulse being detected by the array before the next is transmitted. U.S. Pat. No. 5,521,883 to Fage et al uses a similar phased array to send pulses of different frequencies in different directions and then listen for all echoes simultaneously, thereby decreasing the cycle time. The typical angle of elevation for pulse transmission in the latter systems was between 20 and 30 degrees. The relatively low elevation angle is to enhance Doppler components in the returned echoes due to horizontal (rather than vertical) wind speed in the direction of interrogation.
In recent years, radar DSP (digital signal processing) techniques have been applied to the sodar to achieve improved s/n. In particular, pulse-compression techniques have been used, in which the echoes from a phase or frequency coded pulse are processed with matched filters using Fourier transforms to give the range resolution normally associated with a shorter pulse with a much higher peak power. Such coded pulses are said to have ‘pulse-compression’ waveforms or to be ‘pulse coded’. For simplicity, pulses of this type will be called ‘chirps’. In an article entitled: “Use of Coded Waveforms for SODAR Systems” [Meteorol. Atomos. Phys. 71, 15-23 (1999)], S G Bradley recently reviewed, with simulations, the use of radar pulse compression techniques to improve amplitude discrimination in sodar. Examples of the use of pulse compression techniques in radar can be found in U.S. Pat. No. 6,208,285 to Burkhardt, U.S. Pat. No. 6,087,981 to Normat et al, and U.S. Pat. No. 6,040,898 to Mroski et al. Despite the application of such sophisticated techniques to sodar, a review by Crescenti entitled, “The Degradation of Doppler Sodar Performance Due to Noise” [Crescenti, G. H., 1998, Atmospheric Environment, 32, 1499-1509], found that severe problems remain even at modest ranges of 1500 m.